Variable inductor



P 4, 1956 2 J. F. GORDON 2,762,020

VARIIABLE INDUCTOR Filed April 7, 1953 2 Sheets-Sheet 1 INVENTOR. .mw; /-i GOPZJOA/ %rrom irs Sept. 4, 1956 J. F. GORDON 2 VARIABLE INDUCTOR Filed April 7, 1953 2 Sheets-Sheet 2 I N VEN TOR. ./4M5 A aepalv WWSW Uni i tatS i VARIABLE tNDucTon James E. Gordon, Concord, calili assignor, by mesne assignments, to Helipot Corporation, South Pasadena, Calif., a corporation of California Application April 7, 1953, Serial No. 347,375 12 Claims. or. 336-410) This invention relates to variable inductors, including both self inductors and mutual inductors or transformers. It is particularly applicable to such inductors as utilized irrradio, television, and communication circuits generally, but it is not limited to such uses, being generally applicable in, a large class of cases wherean element having variable seltor mutual-inductance is desired.

Among the objects of this invention are to provide a type of inductor wherein the inductance may be varied continuously from maximum to minimum value; to provide a type of inductor the inductance of whichmay be varied over ranges which may be prede'signed from values of less than 2:1 up to values in the neighborhood of 100:1 or even more; to provide a type of inductor having a minimum of distributed capacity, sotha't with a device having a range of inductance values of the order of 100:1 a tuning range of over :1 may be realized; to provide a type of inductor which has, inherently, the self-shielding properties of a toroid and which may be readily shielded additionally so as to be substantially wholly unaffected by external magnetic or electric fields; to provide inductors which may readily be ganged for concurrent tuning of a plurality of Circuits; to provide atype of inductor which may be adjusted by a simple rotary motion (as by a tuning knob) through 180 degrees; and to provide a type of variable inductor which lends itself to quantity production methods and is simple and economical to construct. I r r The operation of inductorsof the type herein consid ered depends upon certain well known characteristics of ferro-magnetic materials. The inductance of any circuit is directly proportional to the total magnetic flux linking the circuit when unit current flows therein. Ferro-ma'gnetic materials are characterized by the fact that relative small magnetomotive forces will establish relatively large fluxes therethrough; i. e., for moderate magnetomotive forces the permeability of these mateirals is' high. As the magneto-motive force acting on the material is increased beyond a certain limit, which' is characteris'tic of the specific term-magnetic material, additional increments of magneto-motive force produce progressively smaller and smaller increases in total ilux,'until, at saturation, further increases in magneto-motive force'produce only substantially the same flux increments which would occur were the material replaced by air; i. e.,'tlie permeability at saturation approaches unity. These effects can beexplained on the theory that these materials are" comprised of -a large number of elementary magnets which are normally disoriented, being polarized in random directions, and that when a magneto-motive force is applied thereto these elementary magnets tend to aline so that their individual fluxes are added to the magnetizing flux itself and thus increase the total. If substantially all of the elementary magnets are alined no further increase in the total flux due to this cause can take place and the saturation point is reached;

These related phenomena have been employed 11'1" the past to vary the inductance of a circuit'ora circuit ele- 2,762,020 Fatented Sept. 4-, 1956 ice meat. Variable reactors are known in which a direct Cllll'dl'lt winding and an alternating current winding constitu'ti'ng the circuit whose inductance is to be varied are superimposed upon the same core, preferably in such manner as to be mutually non-inductive. The total average flux through the core is determined by the current flowing in the D. C winding and if this current is powerful enough to keep most ofthe elementary magnets alined the inductance of the device is accordingly reduced. This same principle lies at the basis of the, present invention, but is, however, applied in an entirely dilferen; manner. r

Broadly considered, the inductor of the present invention comprises a core offerro-magnetic material having a winding or windings disposed thereon. In adidtion to the magnetic circuit formed by the core within the windbig, a second magnetic circuit is providedwhich includes a magnet, preferably apermanent magnet, and which comprises relatively moyable portions so disposed as to provide poles on opposite sides; of the ferromagnetic core, whereby in one position ot the movable portions the poles are directly opposite and the magneticcircuit including the magnet includesthat portion of the core directly between the poles and only that portion. The parts of this magnetic circuit are movable, however, in such manneras to traverse at leastone of the poles along thele'n'gth of the core, in the direction of the axis of th'e coil wound thereon, so thatgreater andgrjeater proportions of .the total core length are included between the poles, The magnet is of sufficient strength to saturate that portion of the core subtended by the poles, at least to a sufiicient extent materially to reduceits effective permeability and increase the total reluctance in the magneticcircuit linking the winding, hence reduce the total flux induced thereby and the inductance pr sented in any circuit of which the winding forms a part. Preferably the term-magnetic core is of toroidal form and formany purposes it is preferable that it be formed of a ferrite. The additional magnetic circuit may then comprise two magnets, mounted on opposite sides or the core andits winding in planes substantially parallel thereto. One of these magnets may then be in fixed relation with respect to the core and its winding; the other of the magnets mounted for rotation around the axis of the toroid. It will thus be seen that when the two magnets areparallel and with opposite poles facing eachother directly across the core only the portion of thecore directly between thepoles will be saturated and the device will present maximum inductance. When the movable magnet has heenrotated through degrees, however, so thatlike poles face each other across the core, the return paths for ,the flux of both magnets is through the core itself and. the entire body may thereby be saturated, so that with respect to small currents in thewinding the effective inductance as viewed from the winding terminals is sub,- stantially that of an air core coil. Variation in inductance depends upon the permeability of the core itself when unsaturated and the total inductive fiux at which the saturation occurs, but devices of this type have actually. been constructed with' inductance ratios of two orders of magnitude, and with the use of materials which have been described in'the literature but have not, as yet,

Fig. 3 is a plan view of an embodiment of the invention utilizing an annular or toroidal core;

Fig. 4 is a side elevation of the device of Fig. 3;

Fig. 5 is a diagram illustrating the path of the magnetic flux through the core when the opposite magnetic poles of the external magnetic circuit are directly aligned thereacross;

Fig. 6 is a schematic illustration of the magnetic fluxes set up by the external magnetic circuit within the core.

Fig. 7 is a plan view of a toroidal core device embody ing the invention, using a slightly different form of external magnetic circuit from that illustrated in Fig. 3;

Fig. 8 is a side elevation of the device of Fig. 6;

Fig. 9 is an axial sectional view of a toroidal core device utilizing magnets capable of developing a more powerful saturation field than those of the preceding figures;

Fig. 10 is an axial sectional view, similar generally to Fig. 7 illustrating two different types of magnet structure;

Fig. 11 is a schematic diagram illustrating a type of toroidal winding which is particularly useful in the device of this inventon;

Fig. 12 is an illustration of an embodiment of the invention using a single turn toroidal winding;

Fig. 13 is a plan view of the device of Fig. 12;

Fig. 14 is an exploded isometric view of a commercial form of the device.

In the descriptions which follow two elementary forms of the invention will first be described mechanically, following which a principle of operation will be discussed. Thereafter, several modfications of the invention, having slightly different properties and, to some extent, different applications will be considered.

Turning now to the drawings, Figs. 1 and 2 illustrate a form of the device wherein a straight ferro-magnetic core of the open type is used, Fig. 1 being a plan and Fig. 2 an elevation of the device. A ferro-rnagnetic core 1 of rectangular cross-section is shown. Disposed on this core is a magnetic winding 3, which is illustrated in purely schematic form in Fig. 1 and is shown in end elevation in Fig. 2. This core with its winding constitutes the inductor proper. The external or saturating magnetic circuit through which the inductance of the device is varied comprises, in this case, a bar magnet 5, pivoted at one end (designated as S) to a soft iron bar or yoke 7, the magnet and the iron bar being separated by a soft iron or mild steel thimble 9 of such length that the magnet and bar are separated by substantially the distance across the core and its winding. A pivot pin 11 passes through magnet, bar, and thimble so that the angle 0 between the magnet and the bar 7 can be varied at will. Thus the two elements mentioned can be brought parallel so that the ends which are in contact, or nearly in contact, with the magnet winding, are opposed directly across the core or they can be spread apart so that they are adjacent to opposite ends of the core as shown in the figure.

The material used for the core 1 may be any of those having ferro-magnetic properties. The constants of such materials vary greatly. For some purposes it may be desirable to use molded powdered iron, or materials wherein pure iron or iron alloys such as permalloy in the form of extremely fine powder, is embedded in a plastic binder. Such materials may have effective permeabilities as low as 2 or 3 because of the relatively large gaps between the magnetic particles. For other purposes the conventional solid ferro-magnetic materials may be used, such as soft iron, silicon steel, or the iron-nickel-cobalt alloys known as the permalloy group. Where these materials are used it is desirable that they be in laminated form, with insulation (which may be in the form of a varnish) between the laminations. For the purposes to which this invention is most particularly applicable, however, the ferro-magnetic material known as ferrites are to be preferred. These are oxides bi-valent metals crystallizing in accordance with the cubic system. Particularly when mixtures of such oxides are used they are strongly ferromagnetic and by choice of mixtures and heat treatments a very wide range of magnetic properties can be obtained. The advantage of their use lies in the fact that they have the nature of ceramics; i. e., they are insulators and hence introduce negligible eddy-current losses. Furthermore, the distributed capacity of a coil wound on such an insulator has a much lower value than one wound on a conductive core since there is no conductive bridge between the elementary condensers formed by the various turns of the winding and the core itself. These considerations as to core material hold for all of the cores used in the various types of inductors later to be described.

Figs. 3 and 4 illustrate a second and preferable form of inductor in accordance with this invention. In this case the ferro-magnetic core 13 is of annular or toroidal form. On it is disposed a winding 15. On one side of the core and extending diametrically across it is a fixed permanent magnet 17. On the other side of the core is a second permanent magnet 19 mounted on a shaft 21 which is journaled in suitable bearing (not shown) so that it may be rotated about the axis of the toroid. Both of the magnets are shown as triangular in cross-section, so that each presents a wedge-shaped edge to the core. It may be noted here that the wedge-shaped or chisel edge is not mandatory but it is preferable that the poles presented to the core, whether formed on the magnet itself or by separate pole pieces as will later be described in connection with some alternative forms of the invention, be so shaped as to concentrate the field between magnets and core. The device will operate with broad flat pole pieces of the type illustrated in Fig. 1, which have no provision for concentrating the field. The substitution of cylindrical magnets, which present a more nearly line contact to the face of the core gives better results, but the chisel-edge arrange ment is best of all since it gives the greatest field concentration.

Consider now Fig. 1 and assume that the magnet 5 and iron yoke '7 are brought together in parallel position with the pole N of the magnet and the pole S of the bar opposite each other at the center of the core 1. A magnetic circuit is then formed, excited by the magnet 5, and comprising the magnet, the thimble 9, and the bar or yoke 7 and including the small central portion of the core between the pole of the magnet and the induced pole of the yoke. The transverse fiux will then be induced through the core between the two opposed poles, the permanent pole of the magnet and the induced pole of the yoke. The path of the flux through the yoke will depend to a considerable degree on the shape of the pole pieces. Thus, should cylindrical magnets be used instead of the fiat ones shown in Fig. l, the flux path would be much as is illustrated in Fig. 5. If the magnet be sufficiently powerful this will serve to saturate a small section at the center of the core, and reduce its effective permeability, with respect to any fields induced by the winding 1, to unity. The effect of the inductance on the core is substantially as though a small air gap were introduced at the center thereof. Owing to the divergence of the flux lines between the poles the saturation will not be complete in all portions of the flux path; there will be fringe fields Where the permeability of the core is merely reduced from its initial value, but it can be treated as though the range of reduced permeability merely widened the hypothetical air gaps slightly. The more concentrated the field the narrower the magnetic gap, and through properly shaping pole pieces its effective length may be made very small.

If, now, the magnet and yoke are separated into the position actually shown in Fig. l the magnetic circuit carrying the flux induced by the magnet will include the entire length of the core and if the magnet be strong enough the entire core will be substantially saturated. The inductance of the entire device then becomes substantially what it would he were the core removed entirely. The change in inductance will be a direct function of the maximum permeability of the core material.

The form of the invention shown in Fig. l is not that which is most generally useful, but is illustrated because it makes clearly apparent haw varying lengths ofthe term-magnetic core may be included in an external magnetic circuit, and, further, because it illustrates that the external circuit need not include more than one magnet to produce an operative device, although it will be clear that if desired the yoke '7 could be replaced by a permanentmagnet having its poles at the position indicated.

The form of the invention illustrated in Figs. 3 and 4 is the prototype from which the modifications later to be considered have been developed. The diagram of Fig. 6 illustrates the conditions existing in the operation of the device of Figs. 3 and 4. In Fig. 6 the dimensions of the toroid have been changed materially for the purposes of illustration, but the principles involved are identical with those in the more compact form of the device.

In Fig. 6 the annulus 131 illustrates the toroidal ferromagnetic core, the windings being. omitted for the sake of clarity. The reference characters 1-7-1 and 19 designate magnets corresponding to the magnets 17 and 19 of Figs. 3 and 4.

Considering first only the core 131 and the one magnet 191 and assuming that the latter is sufliciently powerful substantially to saturate the core, when placed in close apposition thereto it will develop in the core flux in the directions illustrated by the arrow heads on the solid line 23.

The magnet I71 is assumed to be identical with magnet' I91. In the position shown the direction of its induced flux will be that indicated by' the arrow heads on the dotted circle 25. If either magnet be rotated the magnetic field which it develops will, of course, rotate with it, so that if'the magnet 191 be rotated into the position shown for the magnet 171 its field in the various portions of the core could be represented by'the arrows on the circle 25 in the same way as is shown Where the magnet '17 i's'in that position.

If, however, the two magnets are both in place we can consider the resultant flux in the core in two Ways, both-of which lead to the same results. By the principle of superposition we can consider both fluxes as existing concurrently in the core. In the segments of the core designated by reference characters A concurrent equal fluxes from the two magnets are in opposite directions and the resultant flux is nil. In the portions B of the core the fluxes are inth'e same directionand the resultant is their sum. If the magnet 191 be rotated counterclockwi'se'until it is in the position shown for the magnet 171 but with'its north pole N immediately above the south pole S of the magnet l71the portions of the core where the fields areopposed will lengthen progressivelywhile the portions B where the fluxes reinforce will be progressively shortened. The result is the same as has already been discussed in connection with Figs. 1 and 2; the portions of the core included in the saturatingmagnetic circuits are changed by changing the relative positions ofthe magnets. This point of view is of value sinceit shows that where the two magnets are of equal strength not saturating field exists between the'like'p'oles otthe magnets.

In accordance with the second point of view the inclusion of varying lengths of the core in the saturating circuit may be more readily apparent. In this case the external magnetic circuits may be traced from the pole N of 'rnagnet W1, counterclockwise through the portion B of the core, to the pole S of the magnet 171, diametrically across the device to the pole N of magnet E71 and then clockwise through the core back to the pole S of magnet 191. From this point of view it is. quite clear that varying lengths of the core are included in this circuit. It is also clear from this that whenopposite poles are directly opposed across the core the situation becomes identical with that illustrated in Fig. the core then all) 6 efiectively becomes an annulus with two magnetic gaps at diametrically opposite position thereacross. Relative rotation of the magnets lengthens these gaps until, when like poles are opposed across the core section the entire core is saturated and the gapsi occupy the entire circumference. The inductance of the coil wound upon the core then approaches that which it would have if the core were removed, leaving an air core toroid What variations in the inductance of the coil can be achieved by the device depends upon the materials available for both the magnets and the core. The magnets obviously should be both strong and permanent; i. e., they should have both high residual magnetism and high coercive force and preferably they should be stabilized so that their strength remains constant indefinitely. Therefore while any permanent magnet material may be used for the purpose the choice is one of the alloys, such as that marketed under the name of Alnicov which possesses the desired characteristics in highest degree. An even wider choice exists in the core materials. Considering the ferrites alone, various grades are oifered for sale by one company having maximum permeabilities varying from 97 up to 4300 and saturating at values of inductiori from 840 gausses fo'r'the type first mentioned to 3400 gaus'ses for the second type. The magnets used must be capable of developing sufiicient flux to cause saturatioti' if the device is to produce the best results. Furthermore, the higher the permeability the greater the rangeof inductance that can be covered by a single device.

It has already been mentioned that when the device is so adjusted that like poles face each other across the core the inductance is substantially that of an air core t'oroid. Iriductance'is determined by the linkages of the flux with the turns of the coil, and since, in a toroidal winding, the flux is to all intents and purposes confiried entirely with that in the toroid in this situation of like opposed poles the flux becomesinversely as the reluctance of the two semi-circular paths around the toroid and the inductance therefore maybe considered as directly proportional to the permeance of those paths, the permeability whereof is substantially unity for the situation considered.

If the position of one magnet be now reversed, and assuming that the permeability of the core material is high-say 1000 or morethe reluctance of the circuit may be'considered as residing entirely in the two magnetic gaps between'the poles, and with chisel-edged pole pieces theelfective length of these gaps may be made'less than one per cent of the means of circumference of the core. The gaps are so smallunder these circumstances that the magnetic held of the coil may still be considered as chtirely confined with the toroid itself.

Under this condition the inductance of the coil he'- comes, at least to a first and close approximation:

Where L isequal to the inductance at a specific setting, a isthe fraction of the circumference which is saturated, and M is theeffective permeability of the core material. The quantity a varies almost exactly linearly with the angle 0 except where O approaches 0' or The reluctance therefore varies substantially linearly, bein g the difference of two linearly varying quantities, and the permeance, and therefore the inductance, varies hyperbolically with the angle between the magnets, except for slight perturbations at the 0 and 180 extremes resulting from minor departures of the fields from the theoretical ideal. The linear variation of the reluctance with position of the magnets assumed by this simplified theory may be approached more or less closely in practice, but. there are-certain correction factors to be considered. The magnetizing field H is always confined within the toroid. The induced field B is not so confined, but may spread into the surrounding space in substantially the same manner as though separate lengths of core material, corresponding in size and shape to the unsaturated portion of the core, existed within the core. Where the saturated portions of the core are short the induced field still lies almost wholly within the coil and the linear relation of the simple theory is approached. As the unsaturated portions become longer the induction field does not link all of the turns of the coil and the situation approaches that of two short solenoids separated in space. If the internal diameter of the toroid is small in comparison with the external diameter the difference from the simple theory may be very small, but if a toroid having relative internal and external diameters such as are illustrated in Fig. 6 were used the difference may be material. When the unsaturated portion of the coil becomes very small the additional path permeability offered by the core also becomes small. The result is a curve departing somewhat from the straight line form and becoming slightly S-shaped, although the curvature of the ends may be reduced to a small value by careful design.

Where the core materials are used having permeabilities only slightly greater than air other factors may supervene further to distort the curve and limit the inductance range offered by the device. It is quite clear that if a material were used having a permeability of only, say 3. the maximum possible inductance range would be 3:1. With low permeability cores, however, the theoretical range may be difiicult to achieve even approximately since the difference in permeability between air paths and core paths for the magnet fiux may result in large leakage fluxes where like poles are opposed across the core. So much fiux may thus be diverted that core saturation, giving the minimum inductance desired, may not be attained. In certain experimental devices using low permeabilities materials for the core the latter effect is so pronounced that the measured inductance becomes substantially constant after the core has been rotated through its first 90 degrees.

The shape of the magnet position-inductance curve also depends upon the shape of the BH curve of the magnetic material used. The most desirable materials are those having Z-shaped hysteresis loops, wherein the induction increases substantially linearly almost to the point of saturation and thereafter remain substantially constant. Naturally, for the purposes mentioned, the smaller the area of the hysteresis loop the better will be the performance of the device, particularly in terms of minimum loss. While the above discussion indicates the parameters affecting the most generally desirable designs, it may, for certain purposes, be adequate or even desirable to depart from the optima indicated in order to achieve a special efiect for particular purposes. Complete theoretical saturation is, in practice, approached asymptotically and while it may very nearly be reached in practice the extent to which it is reached becomes a matter of degree. If the induction in the core from the saturating magnetic circuit is sufficient so that increase of flux results in a decrease of permeability a certain degree of saturation may be said to occur, and inductance may be varied by this means even though the core section included in the magnetic path is far from complete saturation. A specific use may dictate characteristics which would not ordinarily be desirable.

As an illustration of a construction which would not :5

usually be employed but which is still operative, a device as shown in Figs. 7 and 8 may be considered. Here the magnet 27 has pole pieces 29 extending upward to embrace the core 33 across its diameter and the rotating element 31 extends diametrically across the inside diameter of the core 33. For simplicity the winding is omitted in this figure. The element 31 may be either a magnet or a soft iron armature. In either case, when it is alined with the pole pieces 29 the portion of the core between the chisel-edges is saturated as before. If it be of soft iron rotation into the position shown in the figurei. e., through dcgrees-will cause the complete range of change of inductance, whereas if the element 31 be itself a magnet the range will be very much the same as illustrated in Figs. 3 and 4. If it be of soft iron it will be noted that in the position shown it bridges the core at points where the magnetic potential is the same and it will carry no flux, the flux in the core extending bi-directionally around the core between the pole pieces 29. At positions between the 90 degree points the flux will divide non-uniformly between the two halves of the core, again resulting in a non-uniform inductance curve.

Fig. 9 is a cross-sectional view intended primarily to show a desirable design for the permanent magnets. Elements of the invention corresponding to those illustrated in Fig. 4 are designated by the same reference characters with the subscripts 2, the magnets 172 and 192 are substantially identical and are made of Alnico-V by the molding and sintering process. The dotted lines of the poles indicate the chisel-edged shape for giving the concentrated field when the magnets are in the position shown.

Fig. 10 is a similar view showing two other magnet constructions, the parts corresponding to those depicted in Figs. 3 and 4 being designated by the same reference characters with the subscript 3. The magnet structure 173 comprises two short Alnico magnets 29 connected by a soft iron yoke 31. The magnet 193 comprises a bar magnet 33 of Alnico with soft iron pole pieces 35. The Alnico magnets cannot be machined, and therefore they are in each case fitted into holes made for the purpose in the soft iron portion of the circuit.

Fig. 11 is shown primarily to indicate a winding arrangement, and the magnets themselves are therefore not shown. The drawing indicates schematically two separate windings 37 and 37 disposed on the core 39 and laid in opposite direction. Windings of this character have a number of uses; if the terminals shown as extended side by side be connected together, so that the two windings are in parallel, the device will have the same shape of inductance curve as with the single continuous winding shown in Figs. 2 and 3 but the maximum and minimum inductances will be one-quarter of that of a single winding with the same number of turns disposed upon it. The capacity between the end turns is substantially eliminated by this form of winding, however. Furthermore, if the stationary magnet be positioned across the center of the core, at right angles to the diameter to which the leads of the coils are brought out, the potential of the turns bridged by the magnet will be the same and there fore the capacity of these turns to the magnet and hence the effective capacity added thereby to the circuit will be substantially eliminated. The same will be true of the capacity to the movable magnet when it is in its positions of maximum and minimum inductance. This may be important, particularly at the minimum end of the tuning range.

Where the two coils are separate as shown, the device becomes a transformer, of which the winding 37 may be considered as the primary and the winding of 37 as the secondary. For a winding of this type changing the position of the magnets, particularly of the stationary magnet, will make a marked change in the operating characteristic of the transformer. With the relatively stationary magnet in the position shown by the arrow and the movable magnet in the maximum inductance position the coefficient of coupling of the transformer will be substantially unity. Reversing the rotatable magnet will simultaneously decrease the mutual inductance and the coetficient of coupling, the leakage reactance of the transformer increasing to a maximum and the self inductance rising while the mutual inductance falls as the magnets are reversed in relative position. If the device he built as a transformer with two windings placed one on top of the other so that each circumscribes the entire of the winding on the inductor core, is centered within the shield, diametrically across the bottom of the cup formed thereby when the shield is up-ended. Core 69, carrying the winding, schematically indicated at 71, is fitted in on top of the stationary magnet with its leads connected to the terminal 65 and the shield is filled up substantially to the upper surface of the inductor with insulating compound.

The movable magnet 73 is cemented between jaws 75, projecting from a flange 77 on the end of a shaft 79. The shaft rotates in a bushing 81 which is centered in a back mounting plate 83 which fits within the shield 61 and is preferably secured thereto by a thermosetting plastic cement of the same type used for securing the magnet to the shaft. The external end 81 of the bushing 81 is threaded for lock nuts which can be used to hold the device in place on a mounting panel. The usual adjusting knob 85 is provided for tuning or adjusting purposes.

It will be seen that this type of mounting can readily be modified to utilize any of the forms of magnet that have been shown in the various drawings. The Mu Metal shield can be omitted in many cases, and a plastic or non-magnetic case substituted, but the shield is necessary if the device is to be mounted in proximity to strong magnetic fields or if the core be made of some of the high permeability materials which will saturate in the earth's magnetic field. Many other forms of mounting may, of course, be used.

In the particular example illustrated, the core has an inside diameter /2 inch and the mean circumference (length of magnetic path) is 3.41 inches.

The figure of merit Q of the device varies, of course, with the frequency and with the core material used. If the Winding has a low ohmic resistance Q will, in general, increase with decreasing inductance.

Considering the combined etfect of coil resistance and core losses, one sample showed a minimum Q of 50 and a maximum of 75, when measured at 795 kilocycles. Inductance over this range varied in approximately a 10:1 ratio and at this frequency, the maximum Q occurred in the center of the range. At lower values of inductance, which could not, with the equipment available be measured at the lower frequency, the Q varied from a minimum of 50 to a maximum of 75 at the lowest end of the range with high quality ferrite cores, the lowest Q measured was 15 and the highest 75, the lowest Qs being recorded at maximum inductance of higher frequency.

What is claimed is:

l. A variable inductor comprising a ferromagnetic core and a winding disposed thereon, and a magnetic circuit including a magnet and having opposite magnetic poles disposed on opposite sides of the cross-section of said core and relatively movable with respect to each other along said core in the direction of the flux induced in said core by said winding to include varying lengths of said core in said magnetic circuit.

2. A variable inductor comprising a ferromagnetic core forming a substantially closed loop, a winding disposed on said loop, and means for substantially saturating varying portions of said loop magnetically comprising a pair of magnets having poles disposed on opposite sides of the cross-section of said core and relatively movable to bring opposite poles of said magnets directly across from each other transversely of said core in one position, and to subtend increasing portions of said core between them when moved from said position.

3. A variable inductor comprising a ferromagnetic core of annular form, a toroidal winding disposed on said core, a magnet mounted in fixed position extending diametrically across said core on one side thereof, and a second magnet extending diametrically across the other side of said core and mounted for rotation around the axis of the annulus.

4. A variable inductor as defined in claim 3 wherein the material of said core is a ferrite.

5. A variable inductor comprising an annular ferromagnetic core, toroidal windings disposed on said core comprising two coils each subtending substantially onehalf of the circumference thereof, and an external magnetic circuit adapted to direct flux through said core and comprising portions disposed on each side of the crosssection of said core, at least one of said portions comprising a permanent magnet and said portions being relatively movable with respect to each other about the axis of said core to include varying portions thereof in said magnetic circuit.

6. A variable inductor in accordance with claim 5 wherein one portion of said magnetic circuit is fixed relatively to said core in a position spanning substantially the mid points of said coils.

7. A variable inductor as defined in claim 5 wherein said coils are wound in opposite directions and connected in parallel.

8. A variable mutual inductor in accordance with claim 5 wherein said coils are electrically separate.

9. A variable inductor as defined in claim 5 wherein both portions of said magnetic circuit comprise permanent magnets of substantially equal strength.

10. A variable inductor comprising an annular ferromagnetic core and a toroidal winding disposed thereon. magnetic circuit adapted to direct flux through said core and including magnets disposed on opposite sides of said core and poles shaped to concentrate the fiux from said magnets through said core and means for changing the relative positions of said magnets with respect to said core and to each other.

11. A variable inductor as defined in claim 10 wherein said poles are chisel-shaped.

12. A variable mutual inductor comprising a toroidal ferromagnetic core, a plurality of windings disposed on said core, and a pair of mutually movable magnets spanrung said core diametrically of the torus and terminating in poles respectively on opposite sides of the cross-section of said core.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A VARIABLE INDUCTOR COMPRISING A FERROMAGNETIC CORE AND A WINDING DISPOSED THEREON, AND A MAGNETIC CIRCUIT INCLUDING A MAGNET AND HAVING OPPOSITE MAGNETIC POLES DISPOSED ON OPPOSITE SIDES OF THE CROSS-SECTION OF SAID CORE AND RELATIVELY MOVABLE WITH RESPECT TO EACH OTHER ALONG SAID CORE IN THE DIRECTION OF THE FLUX INDUCED IN SAID CORE BY SAID WINDING TO INCLUDE VARYING LENGTHS OF SAID CORE IN SAID MAGNETIC CIRCUIT. 